For a better understanding of the state of the art and the problems relating thereto a method of the conventional type, as known for example from U.S. Pat. No. 5,454,895, for manufacturing a multi-spar box made of fibre-reinforced, polymerizable, thermosetting material (or “composite material”), will be described firstly.
In FIG. 1, a multi-spar box 10 comprises a top panel 11 joined to a bottom panel 12 by means of a series of parallel spars 13a, 13b, 14a, 14b with an I-shaped cross-section (or “H” cross-section), each formed by the joining together of two oppositely arranged elements with a C-shaped cross-section and fillers in the nodal zones. The method envisages using inner tools 15 (so-called “plugs”) which have the function of positioning the spars with respect to the panels 11, 12 of the box and supporting tubular bags 16 which are placed around the inner tools 15 for application of the vacuum. Typically, the inner tools 15 are substantially rigid, box-like structures. The bottom panel 12 is placed on a bottom curing mould 17. The spars 13, 14 already assembled and preformed using the same composite material are arranged in a parallel manner around the inner tools 15, wrapped inside the tubular bags 16. At this point the inner tools 15 with the spars 13, 14 are positioned on the bottom panel 12. The top panel 11 made of composite material, together with a top curing plate 18, is then applied on top of the preformed spars. The entire assembly is enclosed inside a vacuum bag. The top and bottom curing plates 17, 18 are sealed laterally by bags 19. The vacuum is applied to the system thus formed.
During the autoclave polymerization step, pressure is applied by the bag to the outer surfaces of the (top and bottom) panels and to the bases of the spars so as to compact them against the corresponding plates of the mould, while the cores of the spars are compacted by the adjacent bags 16. The cores of the outer spars 13b are acted on externally by means of the lateral bags 19.
The inner tools 15 give the tubular bags 16 their shape so that said shape resembles as far as possible the final shape of the cavity which is to be obtained. The inner tools 15 are designed with dimensions smaller than those of the inner profile of the cavity both so as to be able to receive the tubular bags and so as to ensure that they can be extracted from the structure after the latter has been polymerized. In fact, the inner tools limit the deformation of the assembly formed by each of the pairs of adjacent internal tubular bags and by the core of the spars situated between them. In fact, when, as a result of the heat, fluidification of the resin occurs and the core of the spar could assume any shape, the walls of the adjacent inserts force the membrane formed by the adjacent bags and the core of the spar to remain in the space defined between them.
All the spars of the boxes manufactured using the conventional method described above do not have precise dimensions. In particular, the cores of these spars are subject to bowing (FIG. 2) or tilting (FIG. 3) in an uncontrolled manner, or lateral displacement with respect to the design position (FIG. 4).
The cores of the outer spars act as a support on which various structural elements, such as hinges and supports for the actuators of the movable surfaces and means for fastening the tail unit to the fuselage, are assembled. Therefore, imprecision in the dimensions or position of the cores of the outer spars constitutes a problem since the outer surfaces of bowed, inclined or displaced spars do not provide precise reference surfaces for mechanical connection of the accessories.
With the current manufacturing methods the outer spars have highly variable profiles with variations of up to 3 mm. There exist constructional requirements in respect of assembly, whereby the residual gap between the joining surfaces must be smaller than 0.127 mm; this results in the need to fill the gaps or empty spaces which are bigger than the permitted size by introducing solid fillers or “shims” between the outer surface of the core of the outer spar and the surface of the mechanical connection element. The geometrical form of the shims is specifically defined for each specific joint and cannot be determined beforehand. The construction of the shims, apart from the fact that it cannot be defined before assembly of the parts to be joined together, requires a very expensive manufacturing process. In fact, for each joint it is required to measure the gaps between the parts with great precision, generate their surfaces by means of CAD and part programs, manufacture each dedicated shim by means of numerical-control milling and, finally, check for the necessary fit when installed.